/Engine/Source/Runtime/Core/Public/Misc/PathViews.h
If AppendPath is a relative path, append it as a relative path onto InOutPath. If AppendPath is absolute, reset InOutPath and replace it with RelPath. Handles presence or absence of terminating separator in BasePath. Does not interpret . or ..; each occurrence of these in either path will remain in the combined InOutPath.
static void AppendPath ( FStringBuilderBase & InOutPath, FStringView AppendPath )